type-safe JSON spec and validation tool
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The goal of the library is to provide a type-safe transport layer for serializing and validating JSON. It can be thought of as a subset of json-schema, which is basically a specification of a JSON document. The other goal is getting as much as possible from this specification for free. Right now the following bits are prototyped:

  • All of the instantiations of the transport types structurally follow the schema provided by user
  • Serializers are generic, so they follow from the type-level schema and the're supposed to have a roundtrip property by an implementation.
  • Runtime value validators are generated from the schema. Validation errors are reported as a pairs of a json-path to the element and an error message.
  • There are migrations. It's possible to describe a series of migrations to the schema and have all the necessary machinery to deserialize a user specified version of the schema if there are multiple versions available.
  • Schematic schemas can be exported to json-schema

Be aware that library is experimental and subject to change a lot. The current state can be viewed as a prototype.


$ stack install schematic

GHC Extensions

To use this library without any hassle you should add a few GHC extension either to a module or a cabal file:


Overloaded-extensions are being used only by field combinator, so if you don't use it - feel free to disable it.

GHC Options

I recommend using {-# OPTIONS_GHC -fno-warn-unticked-promoted-constructors #-} pragma in modules which declare schemas and schema migrations, because schematic heavily uses promoted types which should be prefixed with "`" signs and they can be dropped with this option, making it less noisy visually. It's entirely optional and totally opinionated, so feel free to ignore this. Through the documentation, the code is written without any explicit ticks for the same reason. Notice that they're still needed for type-level lists and tuples in schematic code.

Basic Examples

type SchemaExample
  = SchemaObject
    '[ '("foo", SchemaArray '[AEq 1] (SchemaNumber '[NGt 10]))
     , '("bar", SchemaOptional (SchemaText '[TEnum '["foo", "bar"]]))]
  • This schema says that's structurally it's a JSON object which has two fields:
    • field "foo" contains an array of numeric values
    • field "bar" contains an optional text value
  • It also allows you to validate JSON values itself:
    • an array in "foo" contains exactly one element
    • that element must be strictly greater than 10
    • value of "bar" field must be either "foo" or "bar" if presented

This one is valid for example

schemaJson :: ByteString
schemaJson = "{\"foo\": [13], \"bar\": null}"

Also valid

schemaJson :: ByteString
schemaJson = "{\"foo\": [13], \"bar\": \"bar\"}"

it can be parsed and validated like this:

decodeAndValidateJson schemaJson :: ParseResult (JsonRepr SchemaExample)

ParseResult type encodes three possible situations:

  • json is structurally consistent with a schema and runtime validations pass
  • json source is malformed or doesn't correspond to a schema structurally
  • some of the runtime validations have failed, but it's structurally correct overall

It implies a transport layer representation of that data that can be traversed and transformed to whatever internal types user has. It's called JsonRepr. Type parameter is the schema itself.

jsonExample :: JsonRepr SchemaExample
jsonExample = withRepr @SchemaExample
   $  field @"foo" [12]
   :& field @"bar" (Just "bar")
   :& RNil

The most basic and sugar-free way of constructing the same object will look like this:

jsonExample' :: JsonRepr SchemaExample
jsonExample' = ReprObject $
  FieldRepr (ReprArray [ReprNumber 12])
    :& FieldRepr (ReprOptional (Just (ReprText "bar")))
    :& RNil


It's possible to use flens to construct a field lens for a typed object in schematic. Let's suppose you have a schema like this:

type ArraySchema = SchemaArray '[AEq 1] (SchemaNumber '[NGt 10])

type ArrayField = '("foo", ArraySchema)

type FieldsSchema =
  '[ ArrayField, '("bar", SchemaOptional (SchemaText '[TEnum '["foo", "bar"]]))]

type SchemaExample = 'SchemaObject FieldsSchema

There are two ways of working with named fields in the objects:

  • Using the fget and fset functions: fget @"foo" (fput newFooVal objectData) == newFooVal
  • Using the lens library: set (flens @"foo") newFooVal objectData ^. flens @"foo" == newFooVal


Usually migrations are implicit and hard to deal with, schematic deals with it by giving tools to deal with it to a programmer, being as explicit as possible, notifying the developer of a missing transition with a compile-time error. It allows to build versioned HTTP APIs or migrate data from the older versions when reading from the JSON storages.

It's possible to represent schema changes as a series of migrations, which describes a series of json-path/change pairs. Migrations can be applied in succession.

This piece of code will apply a migration to the schema, decode and validate the latest version.

type SchemaExample
  = SchemaObject
    '[ '("foo", SchemaArray '[AEq 1] (SchemaNumber '[NGt 10]))
     , '("bar", SchemaOptional (SchemaText '[TEnum '["foo", "bar"]]))]

type TestMigration =
  'Migration "test_revision"
    '[ Diff '[ PKey "bar" ] (Update (SchemaText '[]))
     , Diff '[ PKey "foo" ] (Update (SchemaNumber '[])) ]

type VS = 'Versioned SchemaExample '[ TestMigration ]

schemaJsonTopVersion :: ByteString
schemaJsonTopVersion = "{ \"foo\": 42, \"bar\": \"bar\" }"

It's possible to decode the latest version like this:

decodeAndValidateJson schemaJsonTopVersion :: ParseResult (JsonRepr (TopVersion (AllVersions VS)))

It's important to differentiate between schema migrations and data migrations. Schema migration is a change of a schema acceptable by a JSON consumer. It transforms one schema to another. Data migrations on the other hand are functions on the data itself. They transform values acceptable by one schema to values acceptable by another. Let's look at the situation when we have to add a field to a JSON Object of a current schema:

type SchemaExample = 'SchemaObject
  '[ '("foo", 'SchemaArray '[ 'AEq 1] ('SchemaNumber '[ 'NGt 10]))
   , '("bar", 'SchemaOptional ('SchemaText '[ 'TEnum '["foo", "bar"]]))]

jsonExample :: JsonRepr SchemaExample
jsonExample = withRepr @SchemaExample
   $ field @"bar" (Just "bar")
  :& field @"foo" [12]
  :& RNil

type AddQuuz =
  'Migration "add_field_quuz"
   '[ 'Diff '[] ('AddKey "quuz" (SchemaNumber '[])) ]

type DeleteQuuz =
  'Migration "remove_field_quuz"
    '[ 'Diff '[] ( 'DeleteKey "quuz") ]

type Migrations = '[ AddQuuz
                   , DeleteQuuz ]

type VersionedJson = 'Versioned SchemaExample Migrations

migrationList :: MigrationList Identity VersionedJson
  =   (migrateObject (\r -> Identity $ field @"quuz" 42 :& r))
  :&& shrinkObject
  :&& MNil

In this instance Migrations is a list of schema migrations and MigrationList Identity VersionedJson is a list of data migrations corresponding to the list of schema migrations: migrateObject (\r -> Identity $ field @"quuz" 42 :& r)) will be used at runtime to transform JsonRepr SchemaExample to JsonRepr (SchemaByRevision "add_quuz" VersionedJson).Identityin the type ofMigrationListis a monad we choose to run our migrations in. If there's need to do database queries or something like that, it can be changed to something more appropriate by a user.migrateObjecttakes a function transforming old fields into new ones,shrinkObject` allows to shrink the JSON object in case migration only removed fields.

How do I construct a value of JsonRepr schema?

JsonRepr schema is a primary representation of a value serializable/deserializable to JSON, but with a twist. It's guaranteed to correspond a type-level schema, it's a type parameter of a same name. It makes constructing it a little bit more involved, but luckily schematic provides a DSL for doing so in a more straightforward fashion. An example would look like this:

type SchemaExample = 'SchemaObject
  '[ '("foo", SchemaArray '[ AEq 1] (SchemaNumber '[NGt 10]))
   , '("bar", SchemaOptional (SchemaText '[TEnum '["foo", "bar"]]))]

jsonExample = withRepr @SchemaExample
   $  field @"bar" (Just "bar")
   :& field @"foo" [12]
   :& RNil

@foo syntax is an explicit type application, which is a feature of GHC 8+, it makes type inference possible without relying on a bunch of Proxys, which makes it syntactically more terse.

GHC Type Applications

schematic provides instances for OverloadedLists and OverloadedStrings GHC extensions to make use of string and list literals for values in a previous example.

Export to json-schema (draft 4)